Method and apparatus for limiting processor clock frequency

ABSTRACT

A method and apparatus for limiting a processor clock frequency includes an overclocking prevention circuit. The overclocking prevention circuit includes a frequency limiting circuit having programmable fusible elements. The frequency limiting circuit outputs a signal identifying a maximum processor clock frequency based on the state of each of the fusible elements. A comparator circuit compares a selected processor clock frequency to the maximum processor clock frequency to determine if the selected processor clock frequency is permitted. If the selected processor clock frequency is not permitted, then the processor is not allowed to operate at the selected clock frequency.

PRIORITY

This application is a continuation of prior application Ser. No.10/640,752, filed Aug. 13, 2003 now U.S. Pat. No. 7,134,037, which is acontinuation of application Ser. No. 10/051,051, filed Jan. 22, 2002,now U.S. Pat. No. 6,633,993, which is a continuation of application Ser.No. 08/990,526, filed Dec. 15, 1997, now U.S. Pat. No. 6,385,735.

BACKGROUND OF THE INVENTION

The present invention relates to processor clock circuits, and moreparticularly, to a method and apparatus for limiting a processor clockfrequency.

Processor manufacturers perform various tests to rate each processor fora particular clock frequency. Tests are typically performed at the waferlevel, and also at the chip level. More restrictive tests can also beperformed at the card level by subjecting the processor to theconstraints of memory and other devices. Based on these tests, themanufacturer can determine the maximum clock speed at which theprocessor can operate without errors. However, many electronicsmanufacturers are very conservative in their clock ratings. For example,a processor that successfully operates during tests at 166 MHz may berated at only 120 MHz.

Moreover, there is a demand for processors across a wide variety ofclock frequencies. As a result, manufacturers typically intentionallyrate processors at frequencies that are significantly lower than theprocessor's maximum clock frequency to meet demand for processors at aparticular frequency. For example, even though 80% of a manufacturer'sprocessors may operate correctly at 300 MHz, many of these processorswill be marked and sold by manufacturers as slower processors (e.g.,133, 150, 166, 200 and 250 MHz) due to market demand for processorsacross a wide variety of clock frequencies.

Because most processors can be clocked at frequencies significantlygreater than their rated (marked) clock frequency, there is presently aproblem with resellers and distributors remarking processors with ahigher frequency and then selling the processors as the higher speedpart to charge a higher price. This is possible because the processorclock speed is typically initialized from the motherboard at reset. Oneor more jumpers on the motherboard can be set to select a processorclock frequency. At system reset, the motherboard outputs frequencyselecting control signals to the processor to select a processor clockfrequency. At reset, the processor samples these frequency selectingcontrol signals. The clock generation logic in the processor thenperforms the appropriate frequency multiplication and division on theexternal clock signal provided from the motherboard to generate theselected processor clock frequency. This allows unscrupulous processorresellers to purchase less expensive processors that are rated at lowerclock frequencies and then remark the processors to a higher clockfrequency. This also allows personal computer manufacturers to overclockthese processors (operate the processor at a clock frequency greaterthan the originally rated frequency) once they are installed in personalcomputers.

There have been attempts to solve the overclocking problem. According toone approach, a maximum processor clock frequency is selected by tyingseveral processor input pins high or low using pull-up and pull-downresistors. However, this hardwiring approach to setting a maximum clockfrequency is susceptible to external manipulation by users andresellers. A user can reconnect these processor input pins to high orlow to select a different clock frequency. As a result, this hardwiringapproach is not secure. Moreover, the hardwiring approach is inflexibleand cumbersome for processor manufacturers because the maximum clockfrequency can be adjusted only by resoldering the pull-up and pull-downresistors. A more flexible approach is desirable.

Therefore, a need exists for a more secure mechanism that preventsresellers and users from operating the processors at clock frequenciesthat are greater than their rated clock frequencies, while providing aflexible technique to allow processor manufacturers to more easilyadjust the maximum clock frequency.

SUMMARY OF THE INVENTION

A method and apparatus is disclosed for limiting a processor clockfrequency. The apparatus includes a frequency limiting circuit includingone or more programmable fusible elements. The frequency limitingcircuit outputs a signal identifying a maximum processor clock frequencybased on the state of each of the fusible elements. The apparatus alsoincludes a comparator circuit coupled to the frequency limiting circuit.The comparator circuit receives a signal identifying a selectedprocessor clock frequency as a first input and receives the signalidentifying the maximum processor clock frequency as a second input. Thecomparator circuit outputs a signal indicating whether or not theselected processor clock frequency is greater than the maximum clockfrequency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a computer according to anembodiment of the present invention.

FIG. 2 illustrates a block diagram of the overclocking preventioncircuit of FIG. 1 according to an embodiment of the present invention.

FIG. 3A illustrates an unprogrammed fuse according to an embodiment ofthe present invention.

FIG. 3B illustrates a programmed fuse according to an embodiment of thepresent invention.

FIG. 4 illustrates a block diagram of the fuse/pin selection circuit ofFIG. 2 according to an embodiment of the present invention.

FIG. 5 illustrates the encoding of the frequency limiting fuse bitsaccording to an embodiment of the present invention.

FIG. 6 is a chart illustrating frequency downbin paths according to anembodiment of the present invention.

DETAILED DESCRIPTION

Referring to the figures in which like numerals indicate like elements,FIG. 1 illustrates a block diagram of a computer according to anembodiment of the present invention. Computer 10 includes a motherboard11 that is connected to a processor 12 via an external clock 14 andprocessor clock frequency selection signals 16. External clock signal 14is generated by motherboard 11 at a predetermined clock frequency. Oneor more jumpers on motherboard 11 may be set to select a particularprocessor clock frequency. The processor clock frequency selectionsignals 16 are output as digital signals at reset by motherboard 11 toidentify the selected maximum processor clock frequency to processor 12.During reset, processor 12 samples processor clock frequency selectionsignals 16. Other techniques can be used to select a processor clockfrequency. For example, the processor clock frequency can be set usingthe Basic Input Output System (BIOS) software.

Processor 12 is coupled to a Cache 18 via a back side bus (BSB) 20.Processor 12 is also coupled to a system bus 22 via a front side bus(FSB) 24. System bus 22 is coupled to an input/output (I/O) device 26via line 27, and to a main memory 28 via line 29. FSB 24 includesaddress, data and control signals. Although not shown in FIG. 1,external clock 14 and processor clock frequency selection signals 16 areprovided from motherboard 11 to processor 12 via FSB 24.

Processor 12 includes a clock generation circuit 30 and an overclockingprevention circuit 32. Clock generation circuit 30 includes frequencydividing and multiplying circuits to generate a processor clock 33.Clock generation circuit 30 receives external clock 14 and processorclock frequency selection signals 16 as inputs, and generates processorclock 33. Processor clock 33 is used internally by the processor 12 toclock various processor circuits. The external clock 14 is provided at apredetermined frequency.

Clock generation circuit 30 generates processor clock 33 based on theexternal clock 14 and the processor clock frequency selection signals16. For example, if processor clock frequency selection signals 16specify a processor clock frequency of 200 MHz and external clock 14 is100 MHz, then clock generation circuit 30 uses a frequency multiplier tomultiply the external clock 14 by two to obtain the 200 MHz processorclock 33.

Processor clock frequency selection signals 16 (as digital signals) canspecify either a particular clock frequency (e.g., 200 MHz).Alternatively, the frequency of processor clock 33 is specified by theprocessor clock frequency selection signals 16 as the ratio of thefrequency of the external clock 14 to the frequency of the processorclock 33 (e.g., 100/200 or ½ in this example).

FIG. 2 illustrates a block diagram of the overclocking preventioncircuit of FIG. 1 according to an embodiment of the present invention.Overclocking prevention circuit 32 includes a frequency selectingcircuit 40, a frequency limiting circuit 42, a comparator circuit 44 anda register 46.

Frequency selecting circuit 40 operates to select a processor clockfrequency. Frequency selecting circuit 40 includes one or moreprogrammable polysilicon fusible elements (frequency selecting fuses) 52that can be programmed to select a processor clock frequency. Accordingto one embodiment of the present invention, there are four frequencyselecting fuses 52. According to an embodiment of the present invention,each fuse 52 outputs a high (or “1”) when programmed (blown), and a low(or “0”) when not programmed. The data from fuses 52 are connected tofuse/pin selection circuit 54 via line 53. Processor clock frequencyselection signals 16 from motherboard 11 are also input to fuse/pinselection circuit 54. The frequency selection signals 16 and the dataoutput from fuses 52 are used by fuse/pin selection circuit 54 to selecta processor clock frequency. Fuse/pin selection circuit 54 outputs theselected processor clock frequency on line 56 to comparator circuit 44.However, frequency selecting circuit 40 is optional. In an alternativeembodiment of overclocking prevention circuit 32 (where circuit 40 isnot present), frequency selection signals 16 are input directly tocomparator circuit 44.

According to one embodiment of frequency selection circuit 40, fuse/pinselection circuit 54 selects and outputs the data from frequencyselection fuses 52 if any of fuses 52 have been programmed (blown).Otherwise (if no fuses 52 are programmed), fuse/pin selection circuit 54selects and outputs the data received from frequency selection signals16. One or more of fuses 52 can be programmed by an electronicsmanufacturer (as an example) to select a processor clock frequency forprocessor 12. However, if no clock frequency is selected using fuses 52(e.g., none of fuses 52 are programmed), then a PC manufacturer or auser is free to select a processor clock frequency via frequencyselection signals 16 (e.g., by connecting the jumpers on motherboard 11to select the maximum clock frequency, or by using the BIOS). However,as described in greater detail below, overclocking prevention circuit 32limits the operation of processor 12 to a permissible range of clockfrequencies (e.g., to a frequency that is less than or equal to themaximum clock frequency).

Frequency limiting circuit 42 includes one or more programmablepolysilicon fusible elements (frequency limiting fuses) 59 and afrequency limit selection circuit 58. According to an embodiment of thepresent invention, there are four frequency limiting fuses 59. Frequencylimiting fuses 59 can be programmed (blown) to select a maximumprocessor clock frequency. According to an embodiment of the presentinvention, each fuse 59 outputs a logic high (“1”) when programmed(blown), and a low (“0”) when not programmed. Frequency limit selectioncircuit 58 receives data from frequency limiting fuses 59 over line 57.Four frequency limit signals 60 are also input to frequency limitselection circuit 58. However, any number of signals 60 can be used.According to an embodiment of the present invention, frequency limitsignals 60 are generated by connecting each of the frequency limitsignals 60 to a pull-up resistor, or a pull-down resistor, to cause eachof signals 60 to be either a high (or “1”) or a low (or “0”),respectively. These resistors can be external to processor 12.

Frequency limit selection circuit 58 operates to select a maximumprocessor clock frequency based on the data from frequency limitingfuses 59 and/or from frequency limit signals 60. According to oneembodiment of the present invention, frequency limit selection circuit58 comprises four OR gates, where each OR gate performs a logical ORoperation on one signal from a fuse 59 and a corresponding signal fromsignals 60. In this manner, a processor manufacturer can use bothpull-up resistors (to set each of frequency limit signals 60 to a high)and/or can program one or more of fuses 59 to select a maximum clockfrequency. Alternatively, the maximum processor clock frequency can beselected by only using frequency limiting fuses 59, or by only usingresistors via frequency limit signals 60. It may be most convenient fora processor manufacturer to select a maximum processor clock frequencyonly by programming one or more of fuses 59 (and not using frequencylimit signals 60).

Comparator circuit 44 receives a selected processor clock frequency online 56 from frequency selecting circuit 40, and a maximum processorclock frequency on line 62 from frequency limiting circuit 42. Accordingto one embodiment, the selected processor clock frequency (on line 56)and the maximum clock frequency (on line 62) are each provided as 4-bitdigital values. Comparator circuit 44 compares the selected processorclock frequency (on line 56) to the maximum clock frequency (on line 62)to determine if the selected processor clock frequency is a permissibleclock frequency. According to an embodiment, the selected processorclock frequency is permissible if it is less than or equal to themaximum clock frequency. Comparator circuit 44 outputs a legal frequencysignal on line 64 indicating whether the selected clock frequency ispermissible. According to an embodiment of the present invention,comparator circuit 44 outputs a high or a “1” if the selected clockfrequency is impermissible, and outputs a low or “0” if the selectedclock frequency is permissible.

The “1” or “0” output from comparator 44 can then be stored in register46. During reset, processor 12 reads the value stored in register 46 todetermine if the selected processor clock frequency is permissible. Ifthe selected processor clock frequency is permissible, the processor isallowed to proceed or operate normally. However, if the selected clockfrequency is impermissible, the processor 12 is not permitted to operateat the selected clock frequency.

Several different actions can occur to prevent the processor 12 fromoperating at an impermissible clock frequency. According to oneembodiment of the present invention, if the selected clock frequency isimpermissible, processor 12 can be simply halted or placed in an endlessloop. This action prevents overclocking of processor 12. According toanother embodiment, if the selected clock frequency is impermissible,processor 12 can automatically operate processor 12 at a clock frequencythat is less than or equal to the maximum clock frequency (and ignorethe clock frequency selected by the user). This allows processor 12 tocontinue operating, but prevents overclocking of processor 12.

The overall operation of overclocking prevention circuit 32 (FIG. 2)according to an embodiment of the present invention will now be brieflydescribed with an example. During chip testing, a processor manufacturerdetermines that processor 12 can operate without errors up to a clockfrequency of 300 MHz. The processor manufacturer does not select aspecific clock frequency (e.g., none of fuses 52 are programmed), butthe manufacturer does set a maximum clock frequency to preventoverclocking and remarking of processor 12. The processor manufacturerprograms one or more of frequency limiting fuses 59 to set the maximumprocessor clock frequency to 200 MHz for processor 12 to beconservative. Frequency limit signals 60 are not used. Processor 12 ismarked and sold as a 200 MHz part. (The 300 MHz part is more expensive).Therefore, the PC manufacturer or user is free to select a processorclock frequency less than or equal to 200 MHz. This may be performedusing one or more jumpers on motherboard 11 or through the BIOS. In thisexample, the user selects a processor clock frequency of 250 MHz.

During reset, the selected processor clock frequency (250 MHz) isprovided as digital data by the motherboard 11 and sampled by processor12 on frequency select signals 16 and is provided as an input tocomparator circuit 44. The maximum clock frequency (digital data fromfuses 59 indicating a maximum frequency of 200 MHz) is provided as aninput to comparator circuit 44. Comparator circuit 44 compares theselected frequency (250 MHz) to the maximum clock frequency (200 MHz)and determines that the selected frequency is impermissible. A “1” iswritten to register 46 to indicate that the selected clock frequency isimpermissible (e.g., is greater than the maximum clock frequency).During reset, processor 12 reads the register 46, and then haltsprocessor 12 because the user has attempted to overclock processor 12.

FIG. 3A illustrates an unprogrammed fuse according to an embodiment ofthe present invention. FIG. 3B illustrates a programmed fuse accordingto an embodiment of the present invention. In FIG. 3A, an upper end offuse 59 is coupled to a power supply voltage (e.g., Vcc) via a pull-upresistor 72. The lower end of fuse 70 is connected to ground. Fuse 59can be programmed (or blown) by applying a voltage to a fuse input 74.When fuse 59 is unprogrammed (not blown), fuse 59 short circuits theground to the power supply voltage via the pull-up resistor 72 to causethe fuse output 76 to be a low (or “0”). Referring to FIG. 3B, afterfuse 59 has been programmed (or blown), the output 76 becomes a high (ora “1”). Once a fuse is programmed (or blown), the fuse output ispermanently set to a “1” and cannot be unprogrammed. Other fusestructures can be used. Also, other materials (other than polysilicon)can be used to construct the fuses.

FIG. 4 illustrates a block diagram of the fuse/pin selection circuit ofFIG. 2 according to an embodiment of the present invention. Fuse/pinselection circuit 54 includes a four-input OR gate 78 and a multiplexer(MUX) 80. The outputs 53 of the four frequency selection fuses 52 areinput as the “1” input to MUX 80, and are each input to OR gate 78. Theoutput 79 of OR gate 78 is input as the select signal for MUX 80.Processor clock frequency selection signals 16 are input as the “0”input to MUX 80. In operation, MUX 80 outputs the selected clockfrequency on line 59 as either the frequency selection signals 16 or thedata output from fuses 52 based on the select input. If any of the fuses52 have been programmed, then the fuse output for the programmed fusewill be a “1”, causing the output of OR gate 78 to be a “1.” In suchcase, the fuse data from fuses 52 are selected by MUX 80 to provide theselected clock frequency. Otherwise (if none of fuses 52 areprogrammed), the output of OR gate 78 will be a “0” and MUX 80 selectsthe frequency selection signals 16 to provide the selected clockfrequency.

According to an alternative embodiment of the present invention, themaximum processor clock frequency can be expressed as the ratio of thefrequency of the external clock 14 to the maximum frequency of theprocessor clock 33 (the “Bus Ratio”). It may be convenient to expressthe maximum processor clock frequency in terms of the Bus ratio,particularly where the external clock 14 is provided from motherboard 11as one of several external clock frequencies. According to an embodimentof the present invention, the external clock frequency can be selectedby motherboard 11 (via an external clock select signal) as either a 66MHz external clock frequency, or a 100 MHz external clock frequency. Theexternal clock select signal can be provided from motherboard 11 to thefrequency limiting circuit 42 to allow circuit 58 to identify thecorrect maximum clock frequency based on the Bus Ratio and the externalclock select signal.

FIG. 5 illustrates the encoding of the frequency limiting fuse bitsaccording to an embodiment of the present invention. The frequencylimiting fuse bit values are shown in the first column and indicate theprogrammed fuse bit data for the frequency limiting fuses 59 fordifferent maximum processor clock frequencies. For each encoded fuse bitvalue (indicated in the first column), the second and third columnsindicate the Bus ratio (and maximum processor clock frequency inparentheses) for the 66 MHz and the 100 MHz external clock frequencies,respectively. For a fuse bit value of “0000” (e.g., where none of fuses59 are programmed), there is no Bus ratio limit (e.g., there is nomaximum processor clock frequency).

FIG. 6 is a chart illustrating frequency downbin paths according to anembodiment of the present invention. As shown in FIG. 6, fuse bits forfrequency limiting fuses 59 can be successively programmed tosuccessively decrease the maximum processor clock speed, or “downbin”the processor to a slower frequency. In FIG. 6, under each fuse bitvalue, the left hand clock frequency indicates the 100 MHz path, and theright-hand clock frequency indicates the 66 MHz path. The arrowsillustrate how maximum processor clock frequency (or the Bus ratiolimit) can be decreased by programming a single fuse in each step alongeach downbin path.

Initially, at the first step of the downbin path, the frequency limitingfuses 59 are all unprogrammed, which is indicated as “0000”, where thereis no limit (e.g., no maximum processor clock frequency). According toan embodiment of the present invention, there are 4 fuse bits. When fusebit 3 is programmed, this fuse bit value (fuse bit value of 1000)corresponds to entry 8 in FIG. 5, which indicates a Bus ratio limit of2/11 (and a 366 MHz maximum processor clock frequency) for the 66 MHzFSB clock frequency, and indicates a Bus ratio limit of 2/9 (and a 400MHz maximum clock frequency). Other steps of the downbin path of FIG. 6similarly correspond to other entries of FIG. 5.

As shown in FIG. 6, after reaching step “1000”, either of the 3unprogrammed fuse bits can then be programmed to proceed to one of thethree next possible steps (x100, x010, and x001, where x indicates adon't care bit value) along the downbin path. As shown in FIG. 6, themaximum processor clock frequency can be decreased to successively lowerclock frequencies by programming additional fuses 59 (e.g., successivedownbinning).

The fuse bit encoding illustrated in FIG. 5 for frequency limiting fuses59 and the downbinning paths of FIG. 6 have several advantages. First,only one fuse must be programmed to select the next lower maximumprocessor clock frequency. For example, during a first series ofprocessor tests, a first maximum clock frequency is set by programmingone fuse. Successive and more stringent tests indicate that theprocessor can successfully operate only at lower frequencies. After eachof these tests, one or more additional fuses can be programmed tosuccessively select the lower maximum clock frequencies. Second, thefuse bit encoding illustrated in FIG. 5 prevents resellers and usersfrom increasing the maximum clock frequency through the programming ofadditional fuses. As shown in FIGS. 5 and 6, regardless which maximumclock frequency is selected, programming additional fuses can onlyselect a lower clock frequency. The fuse bit encoding according to anembodiment of the present invention does not allow the selection of ahigher maximum clock frequency by programming additional fuses.Therefore, even if a reseller knows how to program fuses 59, thereseller cannot increase the maximum processor clock frequency byprogramming additional fuses 59. Rather, the reseller will only decreasethe maximum processor clock frequency by programming additional fuses59.

In addition, the fuse bit encoding of FIG. 5 and the downbinning pathsof FIG. 6 (used for the frequency limiting circuit 42 and fuses 59) cansimilarly be used for frequency selecting circuit 40 and for frequencyselecting fuses 52.

As described above, the present invention includes an overclockingprevention circuit 32 for preventing the overclocking of a processor.The maximum clock frequency output by frequency limiting circuit 42 isprovided to comparator circuit 44 for comparison to the selectedprocessor clock frequency. If the selected processor clock frequency isimpermissible (e.g., if the selected clock frequency is greater than themaximum clock frequency), processor 12 is not permitted to operate atthe selected clock frequency. As a result, the present invention canprevent processor overclocking.

The present invention provides a mechanism for preventing overclockingthat is both secure and flexible. A selected maximum processor clockfrequency can be selected by programming one or more of frequencylimiting fuses 59 (and/or by using signals 60). This mechanism is securebecause the maximum clock frequency cannot be easily manipulated. Inparticular, programming of additional fuses can only decrease themaximum clock frequency. Moreover, the fuse bit encoding according to anembodiment of the present invention allows successive processordownbinning by successively programming additional frequency limitingfuses 59.

Several embodiments of the present invention are specificallyillustrated and/or described herein. However, it will be appreciatedthat modifications and variations of the present invention are coveredby the above teachings and within the purview of the appended claimswithout departing from the spirit and intended scope of the invention.

1. An apparatus comprising: clock generation logic to generate a clocksignal having a clock signal frequency to clock a circuit; andoverclocking prevention logic to prevent operation of the circuit at afrequency greater than a programmable frequency wherein the overclockingprevention logic comprises: a first plurality of programmable fusibleelements programmed with a selected processor clock frequency, theselected processor clock frequency based on the state of each of thefirst plurality of fusible elements, and a comparator and a secondplurality of programmable fusible elements programmed to identify amaximum processor clock frequency based on the state of each of thesecond plurality of fusible elements, the comparator to compare theprogrammed maximum clock frequency to the selected processor clockfrequency and to generate a signal indicating whether or not theselected processor clock frequency is greater than the maximum clockfrequency.
 2. The apparatus according to claim 1, the overclockingprevention logic further comprising: a switching device, the switchingdevice to receive outputs from the second plurality of programmablefusible elements and a multi-bit input signal, the switching device tooutput a signal identifying the selected processor clock frequency. 3.The apparatus according to claim 1 further comprising: a storage device,the storage device to store the signal indicating whether or not theselected processor clock frequency is greater than the maximum clockfrequency.
 4. The apparatus according to claim 1, further comprising: aclock frequency selection circuit, the clock frequency selection circuitto output signals identifying the selected processor clock frequency. 5.A method comprising: identifying a programmed clock frequency, theprogrammed clock frequency based on the state of each fusible element ofa first plurality of programmable fusible elements; comparing a selectedclock frequency to the programmed clock frequency, wherein the selectedclock frequency is based on the state of each fusible element of asecond plurality of programmable fusible elements; and selecting thelower of the selected clock frequency and the programmed clock frequencyto clock a circuit.
 6. The method of claim 5, further comprising:generating a signal indicating whether or not the selected clockfrequency is greater than the programmed clock frequency.
 7. Anapparatus comprising: overclocking prevention logic to prevent operationof a circuit at a frequency greater than a programmable clock frequency,the overclocking prevention logic comprising: a first plurality offusible elements to determine the programmable clock frequency based onthe state of each fusible element of the first plurality of programmablefusible elements, a second plurality of fusible elements to determine aselected clock frequency based on the state of each fusible element ofthe second plurality of programmable fusible elements, and a comparatorto compare the programmable clock frequency to a selected processorclock frequency and to generate a signal indicating whether or not theselected processor clock frequency is greater than the programmableclock frequency.
 8. The apparatus of claim 7, the overclocking logicfurther comprising a switching device to receive outputs from the secondplurality of programmable fusible elements and a multi-bit input signaland to output a signal identifying the selected processor clockfrequency.
 9. The apparatus of claim 7, further comprising: a storagedevice to store the signal indicating whether or not the selectedprocessor clock frequency is greater than the programmable clockfrequency.
 10. The apparatus of claim 7, further comprising: a clockfrequency selection circuit to output signals identifying the selectedprocessor clock frequency.